18. He will go out with his friends. He will pass the exam.
→ If  He don't go out with his friends. He will pass the exam.
19. Without your help, I couldn't find the way to school.
→ If you hadn't helped, I couldn't have found the way to school.
20. But for her busyness, she would come to the class.
If she hadn't be busy, sshe would have come to the class.